Certain carbides, nitrides, borides, oxides, phosphates and suicides exhibit enhanced mechanical properties, including enhanced strength, oxidation resistance, damage tolerance and wear resistance. As a result, these materials have found use in high temperature applications where the materials are subject to extreme temperatures (greater than 3000° C.), as well as corrosive environments. For example, many of the carbides, nitrides, borides, oxides, phosphates and suicides of the elements from Groups IVb, Vb, and VIb of the periodic table, as well as carbides, nitrides, borides, oxides, and silicides of boron, aluminum, and silicon have been used in industrial and other applications where such conditions are likely to be encountered. Generally, structures formed of these materials exhibit improved strength and hardness at ambient and elevated temperatures, improved toughness and wear resistance, high melting points, thermal shock resistance, and oxidation resistance.
Historically, ZrB2 and HfB2 based materials have been the choice for high-temperature ablation resistance in oxidizing environments. They have high melting points (about 3000° C.), excellent oxidation resistance, elevated temperature creep resistance, and moderate resistance to thermal shock. The addition of SiC boosts their resistance to oxidation at intermediate temperatures to produce the best performing diboride material. Above 2200° C. it is the high melting point carbides of Zr, Ta, and Hf (3540° C., 3880° C., and 3890° C., respectively) and not the diborides that exhibit the best oxidation resistance. TaC—HfC solid solutions (e.g. 80% TaC−20% HfC) have high melting temperatures and even better oxidation resistance than the individual Hf and Ta carbides. However, the use of these monolithic materials has been limited due to their poor resistance to thermal shock.
As a more specific application, materials capable of withstanding high temperatures are desired for use in X-ray system design, particularly for the X-ray target. The maximum X-ray power output from an X-ray tube is an important parameter in the operation and maintenance of a radiological system. The time period required to inspect an object is inversely proportional to the X-ray power output. For a given X-ray power output of the X-ray tube, the tube lifetime is directly proportional to its maximum power rating. Accordingly, higher values for the maximum X-ray power output are desirable to reduce the inspection times and the throughput of patients or objects examined with the radiological system, as well as to reduce the maintenance and operating costs as a result, in part, of the longer tube lifetimes. Because of the inefficiencies related to X-ray sources, storage and movement of waste heat from the radiation source is an important consideration in the design of X-ray systems. The thermal expansion match between the substrate and the target material and the ability of the target material to contribute to the high voltage stability are important material characteristics to be considered when designing an X-ray target.
Target materials for X-rays have been made of Cu or similar materials and cooled with circulating oil or water. Other targets utilize standard carbon backed metal targets, which provide improved performance compared to Cu-based targets by eliminating the required cooling but have the disadvantage of an inability of the braze composition to withstand the temperature profiles that are experienced during operation. Where Cu or similar targets with low melting temperatures are used, active target cooling is required to withstand the high temperature during operation, thereby increasing the complexity.
There remains a need for materials exhibiting improved strength, hardness, thermal shock resistance, oxidation resistance and fracture toughness, as compared to presently known materials, for use in high temperature applications and/or harsh chemical environments.